This invention relates to film of polymers produced from a monomer consisting essentially of ethylene. In another aspect, the present invention relates to polyethylene film having a good balance of physical, processing, and optical properties.
In its broadest sense, the term xe2x80x9cfilmxe2x80x9d as used herein refers to self-supporting materials having a wide range of thicknesses. Examples would include thicknesses in the range of 0.05 to about 40 mils, more typically about 0.25 to about 5 mils (1 mil equals {fraction (1/1000)} of an inch). Films can be made using a variety of techniques such as casting, blowing, and extrusion.
Good clarity in polyethylene blown film as indicated by low Haze and high Gloss has been noted in the past to be dependent upon several factors. Typically the Haze increases (and the Gloss decreases) as the polymer density and molecular weight distribution increases. Also, it has been noted that typically the surface roughness increases as the molecular weight distribution and density increases. Film stiffness on the other hand, which is often a desired property of the blown film dependent upon the actual application, has been noted to increase as density increases. Therefore, there has usually been a trade-off between film clarity and stiffness in polyethylene blown film.
Often in forming multi-layered films, a base layer of high molecular weight high density polyethylene or medium molecular weight high density weight polyethylene has been employed to provide strength and a low density polyethylene or linear low density polyethylene layer has been provided to provide other properties. Often, however, it has been noted that the low density polyethylene and linear low density polyethylene layers are tacky and sticky unless antiblock agents are included. Such antiblock agents, however, generally also have an adverse effect upon the clarity and physical properties.
An object of the present invention is to provide a method for producing films of ethylene polymers having a density of at least about 0.925 g/cc which have a good balance of processing, physical, and optical properties.
Other aspects, objects, and advantages of the present invention will be apparent from the following comments.
In accordance with the present invention, there is provided a unusually clear self-supporting film comprising at least one layer having a percent haze of less than 17.8 wherein the polymer of said layer consists essentially of polyethylene having a density of at least about 0.925 g/cc and a molecular weight distribution of no more than 4. The narrow molecular weight polyethylene having a density of at least about 0.925 g/cc is preferably selected from polyethylenes which can be formed into a 1 mil blown film having a percent haze of less than 17.8, or most preferably no more than 10.
In one preferred embodiment the film has only one layer of polymer consisting essentially of polyethylene having a density in the range of 0.93 to about 0.945 g/cc and a molecular weight distribution in the range of about 1.5 to about 4, or more preferably about 1.5 to about 3.5. In another preferred embodiment the film is multilayered and at least one layer has a percent haze of less than 17.8, more preferably a percent haze of less than 10, and comprises polyethylene having a density of at least 0.925 g/cc and a molecular weight distribution no more than 4.
The polyethylene useful for producing the inventive films can be produced using a suitable metallocene-containing polymerization catalyst system. In a particularly preferred embodiment the polyethylene is produced in a slurry, i.e. particle form, type process wherein the polymer is formed under conditions such that the polymer is produced in the form of solid particles that can be readily separated from the liquid polymerization diluent. In such particle form polymerizations it is preferable that the metallocene-containing catalyst system be employed in a form that is substantially insoluble in the polymerization diluent during the polymerization process. Various techniques are known for producing such relatively insoluble catalyst systems. Some examples are shown in U.S. Pat. Nos. 5,354,721; 5,411,925; and 5,414,180.
One particularly preferred type of relatively insoluble solid metallocene catalyst system can be produced by prepolymerizing a mixture of a metallocene, preferably a metallocene having olefinically unsaturated substituents, and a suitable cocatalyst in the presence of an olefin, generally containing 2 to 8 carbon atoms. In particularly preferred embodiment the solid catalyst system is obtained by polymerizing ethylene in the presence of an alkane liquid diluent under slurry polymerization conditions using a special type of metallocene-based catalyst system. The catalyst system is a solid catalyst prepared by (a) combining 5-(9-fluorenyl)-5-(cyclopentadienyl)-hexene-1 zirconium dichloride and methylaluminoxane in a liquid, (b) prepolymerizing ethylene in the resulting liquid, and (c) separating the resulting solid prepolymerized catalyst system from the liquid. It is preferred that the liquid employed in step (a) be an organic liquid in which the methylaluminoxane is at least partially soluble. Preferably some aromatic solvent is employed in step (a). Examples of aromatic solvents include benzene, toluene, ethylbenzene, diethylbenzene, and the like. Preferably the amount of the liquid should be such as to dissolve the product of reaction between the metallocene and the aluminoxane, provide desirable polymerization viscosity for the polymerization, and to permit good mixing. During the mixing, the temperature would preferably be kept below that which would cause the metallocene to decompose. Typically the temperature would be in the range of about xe2x88x9250xc2x0 C. to about 150xc2x0 C. Preferably, the metallocene, the aluminoxane, and the liquid diluent are combined at room temperature, i.e. around 10xc2x0 C. to 30xc2x0 C. The reaction between the aluminoxane and the metallocene is relatively rapid. The reaction rate can vary over a wide range, however, it is generally desired that they be contacted for an amount of time in the range of about 1 minute to about 1 hour.
It is also within the scope of the invention to carry out the step (a) in the presence of a particulate solid. Any number of particulate solids can be employed. Typically this solid would be any inorganic solid that does not interfere with the desired end results. Examples include porous supports such as talc, inorganic oxides, resins to support material such as particulate polyolefins. Examples of inorganic oxide materials include metal oxides of Groups II-V, such as silica, alumina, silica-alumina, and mixtures thereof. Other examples of inorganic oxides are magnesia, titania, zirconia, and the like.
If a solid is employed, it is generally desirable for the solid to be thoroughly dehydrated prior to use. Preferably it is dehydrated so as to contain less than 1 percent loss on ignition. Thermal dehydration may be carried out in a vacuum or while purging with a dry inert gas such as nitrogen at a temperature of about 20xc2x0 C. to about 1000xc2x0 C. and preferably from about 300xc2x0 C. to about 870xc2x0 C. Pressure considerations are not viewed as critical. The duration of the thermal treatment can be from about 1 to about 24 hours as needed.
Dehydration can also be accomplished by subjecting the solid to a chemical treatment in order to remove water and reduce the concentration of surface hydroxyl groups. Chemical treatment is generally capable of converting all water hydroxyl groups in the oxide surface to relatively inert species. Useful chemical agents are for example, carbon monoxide, carbonyl sulfide, trimethylaluminum, ethyl magnesium chloride, chloro silanes such as SiCl4, disilazane, trimethylchlorosilane, dimethylamino trimethylsilane, and the like.
The amount of aluminoxane and metallocene used in forming a liquid catalyst system for the prepolymerization can vary over a wide range. Typically, however, the molar ratio of the aluminum in the aluminoxane to the transition metal of the metallocene is in the range of about 1:1 to about 20,000:1, more preferably a molar ratio of about 50:1 to about 2,000:1 is used. If a particulate solid, i.e. silica, is used, generally it is used in an amount such that the weight ratio of the metallocene to the particulate solid is in the range of about 0.00001/1 to 1/1, more preferably 0.0005/1 to 0.2/1.
The prepolymerization is conducted in the liquid catalyst system, which can be a solution, a slurry, or gel in a liquid. A wide range of olefins can be used for the polymerization. Typically, however, the prepolymerization will be conducted using an olefin, preferably selected from ethylene and non-aromatic alpha olefins, such as propylene. It is within the scope of the invention to use a mixture of olefins, for example, ethylene and a higher alpha olefin can be used for the prepolymerization. The use of a higher alpha olefin, such as 1-butene, with ethylene, is believed to increase the amount of copolymerization occurring between the olefin monomer and the olefinically unsaturated portion of the metallocene.
The prepolymerization can be conducted under relatively mild conditions. Typically this would involve using low pressures of the olefin and relatively low temperatures designed to prevent site decomposition resulting from high concentrations of localized heat. The prepolymerization typically occurs at temperatures in the range of about xe2x88x9215xc2x0 C. to about +150xc2x0 C., more typically in the range of about 0xc2x0 C. to about +30xc2x0 C. The amount of prepolymer can be varied but typically would be in the range of from about 1 to about 95 weight percent of the resulting prepolymerized solid catalyst system, still more preferably about 5 to about 80 weight percent. It is generally desirable to carry out the prepolymerization to at least a point where substantially all of the metallocene is in the solid rather than in the liquid, since that maximizes the use of the metallocene.
After the prepolymerization, the resulting solid prepolymerized catalyst is separated from the liquid reaction mixture. Various techniques known in the art can be used for carrying out this step. For example, the material could be separated by filtration, decantation, or vacuum evaporation. It is currently preferred, however, not to rely upon vacuum evaporation since it is considered desirable to remove substantially all of the soluble components in the liquid reaction product of the prepolymerization from the resulting solid prepolymerized catalyst before it is stored or used for subsequent polymerization. After separating the solid from a liquid, the resulting solid is preferably washed with a hydrocarbon and dried using a high vacuum to remove substantially all the liquids or other volatile components that might still be associated with the solid. The vacuum drying is preferably carried out under relatively mild conditions, i.e. temperatures below 100xc2x0 C. More typically the prepolymerized solid is dried by subjection to a high vacuum at a temperature of about 30xc2x0 C. until a substantially constant weight is achieved. A preferred technique employs at least one initial wash with an aromatic hydrocarbon, such as toluene, followed by a wash with a paraffinic hydrocarbon, such as hexane, and then the vacuum drying.
It is also within the scope of the present invention to add a particulate solid to the liquid catalyst system after it has been formed and then to carry out the prepolymerization in the presence of that solid. Another option is to add a particulate solid of the type aforementioned after the prepolymerization or after the solid prepolymerized catalyst system has been separated from the liquid.
This resulting solid prepolymerized catalyst system is capable of preparing polymers of ethylene having a fairly wide range of densities. Typically, in preparing the lower density versions, the ethylene is polymerized in combination with a smaller amount, generally less than 20 mole percent, of at least one other alpha olefin, generally containing about 3 to about 10 carbon atoms, examples of which include aliphatic hydrocarbons such as butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, and the like. The solid prepolymerized catalyst system can be employed using slurry polymerization conditions. Typically the polymerization temperature would be selected so as to provide slurry polymerization conditions in the particular liquid diluent selected. Typically the temperature would be in the range of about 20xc2x0 C. to about 130xc2x0 C. With isobutane as the liquid diluent, temperatures in the range of about 60xc2x0 C. to about 110xc2x0 C. have been found desirable. For producing polymers for film applications, it is generally desirable to produce a polymer having a melt index of less than 5. This can be accomplished by adjusting the molar ratio of hydrogen to ethylene in the polymerization process, changing the reactor temperature, and/or changing the ethylene concentration.
When the polymerization is carried out in a continuous loop slurry process, it is generally desirable to include in the reaction mixture a small amount of an antistatic agent. An example of such as antistatic agent is the material sold by DuPont Chemical Co. under the trade name Stadis 450.
In a particle form type polymerization the above described type of catalyst system is capable of producing polyethylene homopolymers and copolymers having densities of 0.925 g/cm or higher with molecular weight distributions of no more than 4 that are useful for making films having percent haze of less than 17.8, especially preferred polyethylenes having densities in the range of 0.925 to 0.95g/cc. The polymers produced in that manner have low flow activation energies, i.e. below about 25 kJ/mole, anda critical shear stress at the onset of melt fracture of less than 4xc3x97106 dyne/cm2. This is considered to indicate that the polymers are substantially linear polymers substantially free of long chain branching. The number of long chain branches in such polymers is considered to be less than 0.01/1000 carbon atoms. The term xe2x80x9clong chain branchingxe2x80x9d as used herein refers to branches having a chain length of at least 6 carbon atoms. A method of determining long chain branching is disclosed in Randal, Rev. Macromol. Chem. Phys., C29 (243), 285-297.
The ethylene polymers produced in a particle form process with that catalyst system are also believed to have a very uniform distribution of short chain branches both at the intramolecular level (monomer sequence distributions along the chain) and at the intermolecular level (monomer distribution between polymer chains of different molecular weights). Homopolymers and ethylene-hexene copolymers produced with such catalysts are particularly unusual in that they contain ethylene branches even though no butene comonomer was employed in the polymerization. It is theorized that butene is formed insitu in the polymerization and that such results in a very uniform distribution of the ethylene branches. The shear stress response of such polymers is essentially independent of the molecular weight distribution.
It is typically desirable to add stabilizers to the polymer recovered from the polymerization process. A number of suitable stabilization packages are known in the art. Stabilizers can be incorporated into the polymer during a pelletization step or by reextrusion of previously produced pellets. One example of a stabilizer would be Irganox(copyright) 1010 antioxidant which is believed to be a hindered polyphenol stabilizer containing tetrakis [methylene 3-(3,5-di tertbutyl-4-hydroxy-phenylpropionate)] methane produced by Ciba-Geigy Corporation. Another example is the PEP-Q(copyright) additive which is a product of Sandoz Chemical, the primary ingredient of which is believed to be tetrakis-(2,4-di-tertbutyl-phenyl)-4,4xe2x80x2 biphenyl phosphonite. Other common stabilizer additives include calcium stearate or zinc stearate. Still other stabilizers commonly used include Ultranox 626 antioxidant which is a product of GE, the primary ingredient of which is believed to be bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite, and Ultranox 627A antioxidant which is believed to be Ultranox 626 containing about 7 weight percent of a magnesium aluminum hydrocarbonate. Such stabilizer additives can be employed in generally any suitable amount. The amounts used are generally the same as have been used for other polyethylene polymers. Often the amounts for each additive is less than 0.2 weight percent based upon the weight of the polymer.
The molecular weight of the polyethylene used to make the inventive film can vary over a wide range. Typically for forming films by blowing it is desirable for the polymer to have a melt index in the range of about 0.1-10 dg/min, more preferably about 0.2-5 dg/min. Generally if the melt index of the polymer is less than about 1, it is often desirable to incorporate a processing enhancing amount of a fluoroelastomer processing aid. One example is the fluoroelastomer sold under the trade name Viton by E. I. DuPont de Nemours and Co. Another example is the fluoropolymer sold under the trade name Dynamar FX-9613 by 3M Company. The amount of fluoropolymer employed can vary over a wide range depending upon the particular results desired. Typically it would be employed in an amount in the range of about 0.01 to about 1 weight percent based upon the weight of the polyethylene. In some cases the fluoroelastomer is employed in form of a masterbatch in which the fluoroelastomer is dispersed in a polymer such as LLDPE copolymer of butene and ethylene. One example of such a material is Ampacet 10919 processing aid masterbatch available from AMPACET Corp.
In some applications it may be desirable to include in the polymer of one or more of the layers a slip/anti-block agent, particularly for layers produced from polymers having a density of less than 0.925 g/cc. Generally such materials are inorganic compounds. Some examples include mica, talc, silica, calcium carbonate, and the like. A typical example would be Ampacet 10430 slip/antiblock concentrate available from AMPACET Corp.
It is also within the scope of the present invention for the polyethylene used to produce the inventive films to contain various other additives normally included in polyethylenes, such as heat stabilizers, weather stabilizers, lubricants, etc, in amounts that do not impact unduly on the objects of the present invention. It is also within the scope of the present invention to blend the required narrow molecular weight polyethylene having a density of at least about 0.925 with other polymers so long as the amount of the other polymers does not unduly detract from the beneficial properties of the required polyethylene, i.e. low haze and good handling properties. Generally the required polyethylene is greater than about 50 weight percent of the polymer, more typically at least 90 weight percent of the polymer, and still more preferably at least about 99.5 weight percent of the polymer.
It is within the scope of the present invention to prepare single layer films having a haze of less than 17.8 using polyethylene having a density of at least 0.925 and a molecular weight distribution of no more than about 4. It is considered that such films can be produced by casting, blowing, or extrusion.
It is also within the scope of the present invention to use such a layer of film to form a multilayered film. The polymers employed in the other layers can be selected from generally any of the polymeric materials generally used in producing films. Thus the other layers need not be limited to polymers of ethylene but could contain other polymers such as propylene-butene copolymer, poly(butene-1), styrene-acrylonitrile resin, acrylonitrile-butadiene-styrene resin, polypropylene, ethylene vinyl acetate resin, polyvinylchloride resin, poly(4-methyl-1-pentene), and the like. Multilayers can be formed using techniques generally known in the art such as, for example co-extrusion.
One particularly preferred example of a multilayered film includes one layer having a percent haze of less than 17.8 comprising a polyethylene having a density in the range of about 0.925 to about 0.945 g/cc and a molecular weight distribution of no more than 4 and another layer comprising a second polyethylene having a molecular weight distribution greater than 4, more preferably greater than 6, and still more preferably greater than 10, such as polyethylenes produced using Phillips chromium catalysts or Ziegler-Natta type catalysts.
For some applications it is also desirable for the polyethylene with the broader molecular weight distribution to have a higher density than the polyethylene having the narrower molecular weight distribution, for example a density of at least about 0.945 g/cc. In a preferred embodiment of this type there are at least three layers and the outer layers have a haze of less than 17.8 percent and comprise a polyethylene having a density in the range of about 0.925 to about 0.945 g/cc and a molecular weight distribution of no more than 4, and and the inner layer comprises a polyethylene having a density of at least about 0.945 g/cc.
In another preferred embodiment there are at least three layers and the outer layers have a haze of less than 17.8 percent and consists essentially of polyethylene having a density in the range of about 0.925 to about 0.945 g/cc and a molecular weight distribution of no more than 4, and and the inner layer comprises polyethylene having a molecular weight distribution of at least 10 and a density of less than 0.93 g/cc, most preferably a density in the range of 0.91 to 0.929 g/cc with a HLMI in the range of about 12 to about 24 dg/min.
The most preferred multilayered films are those in which the multilayered film itself has a percent haze of less than 17.8, even more preferably a percent haze of less than 10. In the currently preferred three layer film the outer layers each have a thickness in the range of about 5 to about 25 percent of the total thickness of the three layered film. A particularly preferred inner layer is one having a thickness equal to about 50 to about 90 percent of the total thickness of the three layered film, with the polymer of that inner layer being a low density linear copolymer of ethylene and 1-hexene produced using a Phillips Cr catalyst in a particle form polymerization process, particularly a copolymer having a density in range of about 0.91 to about 0.929 g/cc, an HLMI in the range of about 12 to 24 dg/min. and a molecular weight distribution greater than 10.
It is also within the scope of the present inventive mutilayered films to have a layer of polyethylene having a broader molecular weight distribution and a lower density than the polyethylene in the layer having a percent haze of less than 17.8, for example one layer could have a percent haze of less than 17.8 and be composed of a polyethylene having a density at least 0.925 g/cc and molecular weight distribution of at least 4 and a second layer could be composed of a polyethylene having a density of less than 0.925g/cc, such as for example a low density polyethylene produced by a high pressure process.
It is also within the scope of the present invention to have a multilayered film in which one layer has a percent haze of less than 17.8 wherein the polymer consists essentially of a polyethylene having a density of at least 0.925 g/cc and a molecular weight distribution of less than 4 and another layer composed of a low density polyethylene having a narrow molecular weight distribution and good clarity. In that case the inventive layer of polyethylene provides stiffness that may not be provided by the lower density polyethylene without detracting from the clarity of the lower density polyethylene as much as would a similar density polymer produced by a Phillips chromium catatalyst or a Ziegler-Natta type titanium-containing coordination catalyst.
A layer having a percent haze of less 17.8 made of a polyethylene having a density of less than 0.935 g/cc typically has a much lower melting point than polymers of the same density and molecular weight produced by conventional transition metal coordination catalysts or Phillips chromium catalysts. If a lower melt temperature layer is desired it may therefor be advantageous to use the polyethylenes having a density in the range of 0.925 to 0.935 g/cc and a molecular weight distribution of less than 4 to form the layer having the haze of less than 17.8.
In a particularly preferred embodiment all the polyethylene layers are polyethylenes produced using metallocene catalysts which have molecular weight distributions of less than 4.